Fiber-composite materials are in principle composed of a matrix material made of plastic and, comprised therein, natural or synthetic, organic or inorganic fibers. Because of the low density of fiber-composite components and their high stiffness and strength, they are widely used in aerospace, in vehicle construction, in mechanical engineering and in plant engineering, and also in sports equipment.
In the sector of continuous-fiber-reinforced fiber-composite components there are many different shaping processes, for example the resin transfer molding (RTM) process, in which laid fiber scrims are inserted into a mold and draped, the mold is closed, and the resin is then injected into the mold. The crosslinking of the resin in the mold is typically brought about via introduction of heat. A difficulty, and therefore a certain restrictive factor, in this process is the cutting-to-size and draping of the laid fiber scrims in the mold. The cutting-to-size of the individual layers of textile to fit the various mold geometries is very time-consuming and costly. Preformable fiber reinforcement systems with good handling properties would be desirable here.
Fiber-reinforced materials in the form of prepregs are already used in many industrial applications because they are convenient to handle and give increased efficiency during processing when comparison is made with the alternative wet-lay-up technology.
Requirements for the industrial use of prepregs are not only good handling, relatively long shelf lives at room temperature, but also relatively short cycle times and prepreg hardening temperatures that are low and relatively energy-efficient.
Resins that have the strength and simultaneously high toughness required for use for the production of composite components are polyesters, vinyl esters, and epoxy systems, and just a few others. These include polyurethane resins, which have the toughness and strength required in particular for use for the production of composite profiles, or for use in the pultrusion process. Polyurethane composites also have superior toughness when comparison is made with vinyl esters, unsaturated polyester resins (UPE) or UPE-urethane hybrid resins.
One of the preconditions placed upon the matrix components of a prepreg system is that the resins in what are known as the B-state are not tacky or are only very slightly tacky, but also that they have not completely hardened; instead, the requirement is that the resin matrix has been merely prepolymerized, i.e. it must remain fusible. “B-state” is therefore the term used for the partially polymerized matrix resin in the prepreg that can be stored but remains reactive. The extent of this partial polymerization is precisely that which renders the prepregs non-tacky or only very slightly tacky to the touch, at higher temperature and optionally under pressure allows remelting, and finally hardening, of the matrix resin. Requirements placed upon the crosslinked resin matrix are high interfacial adhesion in relation to the reinforcing materials and embedment components and where appropriate also in relation to other materials, for example metallic or ceramic materials. Other requirements in the crosslinked state are high chemical stability and heat resistance.
DE 102009001793, DE 102009001806, and DE 10201029355 disclose prepregs based on storage-stable (highly) reactive polyurethane composition. The polyisocyanates used here are in essence aliphatic and either have internal blocking (e.g. in the form of uretdione) and/or have blocking by external blocking agents. The disadvantage is that the hardening temperature is from 120° C. to 200° C., depending on the system, and the hardening time/cycle time is very long, being up to 60 minutes, with resultant high energy costs and production costs. Furthermore, the glass transition temperatures obtained when aliphatic polyurethane systems are used are lower than with aromatic systems.